This invention relates generally to signal transmission in rails of railroads, and more particularly this invention relates to assessing track signal susceptibility to electric power line interference.
Various track signalling systems are in use today on United States railroads including the following:
1. Conventional steady energy d.c. relay track circuits.
2. Coded d.c. track circuits.
3. A.C./d.c. track circuits (rectifier systems).
4. Steady energy double element track circuits (vane relays).
5. Coded energy double element track circuits (phase selective).
6. Low frequency electronic track circuits.
7. Cab signal systems
8. Highway crossing overlay track circuits.
9. Highway crossing motion detection (and predictor) systems.
Heretofore, a major unknown in the analysis of such systems has been the susceptibility of the signalling systems to noise derived from electric power transmission lines in close parallel operation with the tracks. Prior susceptibility investigations have, met with two principal objections, to wit:
1. The tests lacked a recognition of the "systems design approach" required in developing the signalling systems. This can lead to unrealistic conclusions regarding the potential measures that might be taken to overcome susceptibility problems. As an example, the replacement of a very low impedance d.c. track relay with a higher impedance relay that could not satisfy the signalling system functions.
2. The tests did not consider all the factors impacting the measurement of the signalling system susceptibility. The realistic effect of variations in ballast and proper impedance of the track (as a component in the signalling circuit) must be included.
Thus, a need exists for improved testing methods that will simulate real life conditions. It is recognized that the desired output will be a limited number of definite track circuit and grade crossing signalling systems noise susceptibility limits. In reality, of course, there are a very large number of noise limits determined by a host of factors, including (among others):
the type of track circuit under consideration (d.c. relay, electronic, etc.),
the function being performed (train detection, broken rail detection, broken down joint detection),
ballast conditions,
length of track circuit,
state of the circuit (occupied or unoccupied),
shunting sensitivity requirements,
impact of overlay circuits,
nature of the noise,
single rail versus double rail circuits,
end-fed versus center-fed circuits.